Electrostatographic printing operates according to the principles and embodiments of non-impact printing as described, e.g., in "Principles of Non-Impact Printing" by Jerome L. Johnson (1986)-Palatino Press-Irvine Calif., 92715 U.S.A.).
Electrostatographic printing includes electrographic printing in which an electrostatic charge is deposited image-wise on a dielectric recording member as well as electrophotographic printing in which an overall electrostatically charged photoconductive dielectric recording member is image-wise exposed to conductivity increasing radiation producing thereby a "direct" or "reversal" toner-developable charge pattern on said recording member. "Direct" development is a positive-positive development. "Reversal" development is of interest in or when from a negative original a positive reproduction has to be made or vice-versa, or when the exposure derives from an image in digital electrical signal form, wherein the electrical signals modulate a laser beam or the light output of light-emitting diodes (LEDs). It is advantageous with respect to a reduced load of the electric signal modulated light source (laser or LEDs) to record graphic information (e.g. printed text) in such a way that the light information corresponds with the graphic characters so that by "reversal" development in the exposed area of a photoconductive recording layer, toner can be deposited to produce a positive reproduction of the electronically stored original. In high speed electrostatographic printing the exposure derives practically always from electronically stored, i.e. computer stored, information.
A review of different toner development systems is given by Thomas L. Thomson in I.E.E.E. Transactions on Electronic Devices, Vol ED 19, pp 495 to 511.
The toner image obtained on a repeatedly used electrostatographic dielectric recording member is transferred onto a printing stock material, usually paper in the form of a web whereon the toner image is fixed, whereupon the web is cut into sheets containing the desired print frame.
As can be learned from the book "The Physics and Technology of Xerographic Processes" by E. M. Williams (1984), Chapter Ten, p204 and seq, the transfer of developed toner images onto paper proceeds by means of electrical corona devices to generate the required electric field to attract the charged toner from the electrostatographic recording member to the paper. The transfer efficiency of toner onto the receptor paper is not only dictated by the contact of the paper with the toner-laden recording member and the corona voltage but also by the conductivity of the paper and particularly by its water content. Paper is not a simple insulating dielectric, so the electrical properties of plain paper have some influence on toner transfer.
Experiments with a variety of paper types and thicknesses (i.e. weights) have established that heavier papers yield improvement in transfer efficiency. Paper types with high porosity, ie high permeability for gases loaded with ions by corona discharge do not allow an efficient toner transfer. Variation in gas permeability or porosity between different paper types is due to overall thickness, degree of filling with sizing agents such as clays, gloss-improving agents and other treatment agents.
Apart from these agents which form a constant factor for conductivity there is the moisture content which fluctuates with the humidity of the environment.
It has been established that as the moisture content of untreated copy paper increases from about 3 to 10% by weight, the surface resistance of said paper decreases nearly six orders in magnitude. Dry paper has very good electric insulating behaviour so that thereon by corona discharge a fairly high electrostatic charge can be deposited before breakdown takes place. On using dry receptor paper the toner attraction force caused by an electrostatic charge at the rearside of the receptor paper can be built up with reasonable charge. Since the leakage of charges flowing through the receptor paper is a function of moisture content (paper humidity), a careful control of said moisture content will be in favour of toner transfer efficiency, image quality and reproducibility in toner printing results.
A careful control of the relative humidity and temperature of the toner in the development station and more particularly of the environment wherein the development takes place will avoid substantial fluctuations in charge/mass (Q/M) ratio of the individual toner particles, which Q/M ratio substantially determines the optical density of the developed and transferred toner image.
The humidity of the environment wherein corona discharge takes place will also determine the ionisation degree and ion charge deposition. A high humidity seriously influences the functioning of corona discharge devices and too high a humidity may give rise to undesirable electric breakdown phenomena.
The humidity of the printer environment has also been found to have an influence upon dimensional stability of the web. This stability is particularly important in multi-station printers where accurate registration of superimposed images is critical.
According to published European patent application 0 154 041 (AGFA-GEVAERT/De Schamphelaere et al), the temperature of the photoconductive layer of an electrophotographic apparatus has to be kept as constant as possible in order to avoid changes in chargeability and discharge characteristics of the photoconductive layer. In order to obtain reproducible printing results under varying temperature conditions of the photoconductive layer a temperature sensing means in the immediate neighbourhood of the photoconductive layer produces electrical signals that are used in a comparator circuit to control the development regulating bias voltage applied during development to a magnetic brush loaded with carrier-toner mixture.
U.S. Pat. No. 5,034,772 (Susuki, assigned to Canon KK) relates to an electrophotographic apparatus containing a humidity measurement device and means for compensating for image forming condition variations caused by changes in humidity. Said compensating means represent a number of electronic control means and circuits that are activated by a temperature/humidity detection means located near the recording member, but do not create stable temperature/humidity conditions.
Another important aspect inseparable from corona discharge in the air is the formation of ozone, the concentration of which must be kept below a certain level in the neighbourhood of the coronas in order to maintain stable charging capacity and to prevent chemical attack of the applied photoconductive substances and/or organic binder therefor. Moreover, in the surroundings of the printing apparatus the ozone concentration may not surpass a level that could pose a health risk for the operating personnel. Therefore, measures to destroy and/or absorb ozone formed in corona-operated electrostatographic printing machines (particularly for high impression numbers) should preferably be taken.
Printing machines of the type described herein are liable to generate dust, primarily toner dust from the development station, but also paper dust. The presence of dust may seriously influence the image writing systems and the corona discharge devices in the printer. It is therefore desirable to reduce the level of dust in the environment of the printer.